That's a pretext. Because they COULD build a 184HP I6 DI NA 2.2-2.5 with no worse fuel economy / CO2 emission and as quick in 0-100km/h.... it is not a technological issue but the nonsense current Euro fashion /trend would kill it, for bad reasons (like displacement vs registration fee) irrelevant to us , in North America.

Please immerse yourself in the world outside the US.
The CO2 emission of the 320i is a couple of g/km less, so that it falls in "energy class A" instead of B in Europe. This means your lease price is €75/€125 (25% vs 20%) less per month.

BMW is not the only car brand in the world. You have a choice For example, you can buy an American car. But you need to stop at the petrol station a lot more often for a car with the same 1 to 60 time as the 320i